Television receiver operating level control



May 16, 1939. R. URTEL. ET AL TELEVISION RECEIVER OPERATING LEVEL CONTROL Filed Oct. 28. 195$ W050 RHRODl/(FR VIM-'0 AMPL/F/ER RfCE/VER AND AMPL/F/ER 0 L5 E Y mwwa m m5? m m. T w A RR n/ Patented May 16, 1939 UNITED STATES PATENT OFFICE TELEVISION RECEIVER OPERATING LEVEL CONTROL Rudolf Urtel, Robert Andrieu, and Max Geiger,

Berlin, Germany,

assignors to Telefunken Gesellschaft fiir Drahtlose Telegraphic m. b. H., Berlin, Germany, a corporation of Germany 8 Claims. .(Cl. 178-753) This invention relates broadly to means for compensating for variations in the intensity of a received television signal and more particularly to a circuit whereby the effects of fading -etc., on the brightness of a reproduced picture are obviated.

In the transmission of audio-frequency signals for a considerable distance by means of carrier wave radiation, the amplitude of the carrier wave is controlled about an average level by the positive and negative alternation of the audio signal to be transmitted. Such audio signals comprise, for instance, well known speech and music broadcasts. The mean value of the carrier wave amplitude remains constant through a comparatively long interval when transmitting such signals modulated by low frequency Waves. Therefore, the receiving amplifier could be maintained at a constant amplitude by means of what is commonly called electrical volume control arrangements which may utilize the D. C. component of the audio frequency current obtained for controlling the degree of the amplification of the receiving amplifiers. Variations in intensity of the carrier which are obviated in this fashion are produced through fading and/or through different field intensities of the transmitters as Well as more or less through very sharp tuning.

Television radiations, on the other hand, are distinguishable from audio radiations in part by the fact that the mean carrier wave amplitude is rather dependent on the average image brilliancy. As a result, it is not possible to use the same compensation methods as are applicable to speech and music transmission.

Television signals usually consist of a carrier whose amplitude is increased from a point which corresponds to dark on the picture to white for the transmission of video signals and synchronizing ampules are radiated in what is termed in the blacker than black direction. Accordingly, our invention is based upon utilization of the lowest value of the carrier amplitude appearing in the image which, of course, bears a definite relationship to black, and such value is used as a reference in determining the amplifier level of operation.

The invention will best be understood by reference to the figures in which:

Figure 1 is one embodiment of our invention.

Figure 2 is an explanatory curve; and

Figure 3 is an explanatory curve.

Referring to Fig. 1, the input circuit of a screen grid tube I3 has connected in its input circuit a resistor II and a diode I connected in parallel with the resistor. I2 is a coupling condenser and the input circuit itself is adapted to be joined to the receiving means in order that a portion of the incoming signal may be fed thereto. Of course, it is understood that in place of grid screen tube 13, the ordinary triode'might be substituted, and in lieu of using the receiving means joined to the video amplifiers, a. separate receiving means might be joined directly to this input circuit. Joined to the plate circuit of the tube I3 is a resistor I4 and a source of positive potential (not shown). The control grid of the tube I3 is joined to a control grid of a multi-grid tube I5 which in this instance is a hexode. Another grid of tube i5 is joined through a compensating potential I6 to the anode circuit of the tube I3. Joined in the anode circuit of the tube I5 is a resistor I! connected to a source of positive-potential (not shown), connected in parallel with the resistor I7 and the source of voltage is a condenser I8 and a rectifier I9. Joined in parallel to the latter is a resistor 2| across which potentials are developed in accordance with the rectified currents.

A portion of the incoming signal is impressed on the input circuit of tube I3 and the rectifier III is connected in such a fashion that as the amplitude of the incoming carrier increases, the negative bias on the control grid of the tube increases. The control grid of the tube I3 is so biased that the operating point of the tube on its characteristic is such that at normal value of the carrier wave amplitude, and the normal value of the demodulated voltage, only the synchronizing impulses 2| (see Fig. 2) produce a plate current in the tube I3, while the image signals 22 (see Fig. 2) lie below the lower bend of the characteristic of this tube as illustrated in the figure referred to. Accordingly, the plate potential of tube I3 decreases in accordance with the synchronizing impulses.

Referring to Figure 3, the characteristic of the hexode I5 and its operating point on this characteristic are illustrated. The operating point is so chosen that for the lowest value 23 of the carrier amplitude corresponding to the picture signals, plate current having a finite value I1 is developed. For higher values of the picture modulating voltage, the plate current of the tube I5 decreases and for lower values, such as corresponds to the synchronizing impulses, there would ordinarily result an increase in the plate current, one of the control grids of the hexode I5 being connected to the control grid of the tube I3. However, due to the voltage drop in the plate circuit of the screen grid tube i3 which is occasioned by the synchronizing impulses, the passage of current through the hexode during the synchronizing impulses 2| (see Figs. 2 and 3) will be completely blocked. Since a grid of the hexode, which is connected across the compensating voltage source Hi to the anode of the screen grid tube, is biased negatively in accordance with the potential drop in the plate circuit of the aforementioned screen grid tube. Accordingly, it will be seen that the maximum plate current in tube I5 results when the picture signals correspond to the value 23 shown in Fig. 3. This plate current value 11 results in a voltage reduction on the anode of the tube l5 and the alternating component will be rectified by the rectifier [9. The potential at the upper end of resistor 20, therefore, is proportional to the value I1 when choosing the time constant of the condenser I8 and resistor 28 large as compared with the duration of the image. If, however, due to fading, or for other reasons, the carrier wave amplitude decreases, then the lowest carrier wave amplitude appearing in the image will result in the production of a plate current in tube l5 that islarger than the value I1. Accordingly, the potential of the upper end of resistor 20 increases and this increase may be utilized to control the degree of amplification of the high frequency amplifiers in the receiver thus balancing or leveling the amplitude fluctuations in the received signal below a definite value.

What we claim is:

1. In a television system of the type in which picture signals and synchronizing signals are transmitted as a composite signal and in which the picture signal voltage representative of conditions of light is of opposite sense to the synchronizing signal voltage, means for receiving said transmitted signals, means for amplifying said received signals, means for developing a direct current control potential for controlling the operating level of the receiver amplifiers comprising a plurality of thermionic tubes including input and output tubes having anode, cathode and at leastone control electrode, means for rectifying the composite signal, means for impressing the rectified composite signal onto the control circuit of the input tube so that the negative bias on said grid increases, as the amplitude of the picture signal increases, means for maintaining the output tube blocked during synchronizing signal receipt, means for rectifying the variations in the output of the output tube, and means for impressing said rectified output onto the amplifier for controlling its operating level.

2. In a television system of the type in which picture signals and synchronizing signals are transmitted as a composite signal and in which the picture signal voltage representative of conditions of light is of opposite sense to the synchronizing signal voltage, means for receiving said transmitted signals, means for amplifying said received signals, means for developing a direct current control potential for controlling the operating level of the receiver amplifiers comprising a thermionic input tube including, anode, cathode and control electrodes, means for impressing a portion of the incoming signal onto said control electrode circuit, rectifying means in the control electrode circuit for developing a negative, bias which increases as the amplitude of the incoming picture signal increases, an output tube including anode, cathode and at least one control electrode, electrical circuit means joining the control electrode of the input tube to a control electrode of the output tube, means for normally maintaining said output tube at an operating level so that its normal output current bears a definite relationship to the normal value of the lowest picture signal amplitude, rectifying means developing a control potential from the changes in the plate current of the output tube, and means for applying said developed control potential to control the operating level of the receiver amplifier.

3. In a television system in which video signals and synchronizing signals are transmitted as a composite signal, means for receiving said transmitted signal, means for amplifying said received signals, means for rectifying said video and synchronizing signals, means for developing a control potential from said signals, a thermionic vacuum tube having anode, cathode and at least one control electrode, said vacuum tube having said developed signals impressed on a control electrode thereof, a second thermionic tube having anode, cathode and at least one control electrode, means for maintaining said latter thermionic tube in a blocked state during receipt of the synchronizing signals, means for developing a potential from the output of the latter thermionic tube which bears a definite relationship to black in the picture, and means for controlling the level of the receiver amplifier in accordance with said latter developed signal.

4. In a television system in which video signals and synchronizing signals are transmitted as a composite signal, means for receiving said transmitted signal, means for amplifying said received signals, means for rectifying said video and synchronizing signals, means for developing a control potential from said signals, a thermionic vacuum tube having anode, cathode and at least one control electrode, said vacuum tube having said developed signals impressed on a control electrode thereof, a second thermionic tube having anode, cathode and at least one control electrode, means for maintaining said latter thermionic tube in a blocked state during receipt of the synchronizing signals, means for developing a potential from the output, said means comprising a rectifier and a resistor connected substantially in parallel thereto for developing signals bearing a definite relationship to black in the picture, and means for controlling the operating level of the receiving amplifier in accordance with said developed potential.

5. In a television system in which video signals and synchronizing signals are transmitted as a composite signal, means for receiving said transmitted signal means for amplifying said received signals, means for rectifying said video and synchronizing signals, means for developing a control potential from said signals, comprising a resistor connected substantially in parallel with said rectifier, a thermionic vacuum tube having anode, cathode and at least one control electrode, said vacuum tube having said developed signals impressed on a control electrode thereof, a second thermionic tube having anode, cathode and at least one control electrode, means for maintaining said latter thermionic tube in a blocked state during receipt of the synchronizing signals, means for developing a potential from the output of the latter thermionic tube which bears a definite relationship to black in the picture, and means for controlling the level of the receiver amplifier in accordance with said latter developed signal.

6. In a television system in which video signals and synchronizing signals are transmitted as a composite signal in which said synchronizing signals are transmitted in a blacker than black direction, means for receiving said transmitted signal, means for amplifying said received signals, means for rectifying said video and synchronizing signals, means for developing a control potential from said signals, comprising a resistor connected substantially in parallel with said rectifier, a thermionic vacuum tube having anode, cathode and at least one control electrode, said vacuum tube having said developed signals impressed on a control electrode thereof, a second thermionic tube having anode, cathode and at least one control electrode, means for maintaining said latter thermionic tube in a blocked state during receipt of the synchronizing signals, means for developing a potential from the output of the latter thermionic tube which bears a definite relationship to black in the picture, and means for controlling the level of the receiver amplifier in accordance with said latter developed signal.

7. In a television system in which video and synchronizing signals are transmitted as a composite signal, means for receiving said composite signal, means for amplifying said composite signal, means for rectifying a portion of said received composite signal, means for developing a control potential from said rectified signal, a first thermionic vacuum tube having anode, cathode and at least one control electrode, means for impressing said control potential onto a control electrode of the thermionic tube, a second thermionic tube having anode, cathode and a plurality of control electrodes, means connecting one of the control electrodes of said latter tube to a control electrode of the first tube, means connecting another control electrode of the second thermionic tube to the output circuit of the first tube, means for rectifying the output current of the second thermionic tube, means for developing a control potential from the rectified current bearing a definite relationship to picture black, and means for controlling the operating level of the receiver amplifier means in accordance with said latter developed control potential.

8. In a television system in which video and synchronizing signals are transmitted as a composite signal, means for receiving said composite signal, means for amplifying said composite signal, means for rectifying a portion of said received composite signal, means for developing a control potential from said rectified signal, a first thermionic vacuum tube having anode, cathode and at least one control electrode, means for impressing said control potential onto a control electrode of the thermionic tube, a second thermionic tube having anode, cathode and a plurality of control electrodes, means connecting one of the control electrodes of said latter tube to a control electrode of the first tube, means connecting another control electrode of the second thermionic tube to the output circuit of the first tube, means for rectifying the output current of thesecond thermionic tube, means for developing a control potential bearing a definite relationship to picture black, a time constant circuit connected to said rectifier, said time constant circuit having a time constant greater than the duration of one frame of the transmitted picture, and means for controlling the operating level of the receiver amplifier means in accordance with said developed potential.

RUDOLF URTEL. ROBERT ANDRIEU. MAX GEIGER. 

